A weather forecasting device using a barometer which has a pointer to indicate a value of atmospheric pressure at a particular point of time on the dial has already been proposed, wherein an auxiliary pointer is installed coaxially with said main pointer and adapted to be manually moved to overlap said main pointer so that upon lapse of a given period of time, the atmospheric pressure change produced during that time can be known by the difference between the atmospheric pressure values indicated by the auxiliary and main pointers, on the basis of which difference a probable weather change is forecasted. With this conventional weather forecasting device, however, it is only possible to know the difference between the value of atmospheric pressure which exists when the auxiliary pointer is manually set and the value of atmospheric pressure which exists at the present point of time, and it is impossible to know with certainty the present tendency of atmospheric pressure change, that is, whether atmospheric pressure is rising or falling, so that the weather forecast coming-true probability is relatively low.